This invention relates photoluminescent materials (or phosphors) and lighting technology. In particular, this invention relates to phosphors containing borate of terbium, alkaline-earth, and Group-3 metals, and to light sources incorporating such.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert ultraviolet (“UV”) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or X rays (for example, scintillators in X-ray detection systems).
Fluorescent lamps having high luminous output and color rendering indices (“CRI”), which are based on mercury discharge and used for illumination, typically include three phosphors that convert UV radiation of the mercury discharge into relatively narrow bands of blue, green, and red visible light, concentrated in the spectral regions where the human eye has the highest sensitivity (450, 540, and 610 nm). These fluorescent lamps are commonly called tricolor lamps. Although a CRI of about 85 would give a normal appearance to most objects, some typical colors will look unnatural under illumination with tricolor lamps. Therefore, for certain applications, still higher CRIs are very desirable. Effort has been expended to provide efficient green light-emitting phosphors. Traditional green light-emitting phosphors have been LaPO4:Ce3+,Tb3+ and GdMgB5O10:Ce3+,Tb3+, which rely on a sensitization of Ce3+ ions and a transfer of energy from Ce3+ ions to Tb3+ ions. A more recently available green light-emitting phosphor has been (Ce,Tb)MgAl11O19, which also operates on the same principle. Although GdMgB5O10:Ce3+,Tb3+ uses lower amounts of cerium, LaPO4:Ce3+,Tb3+ and (Ce,Tb)MgAl11O19 require considerable amounts of cerium, which contribute to their manufacturing costs. In addition, these phosphors still exhibit some undesirable UV emission from the cerium ions. Therefore, there is a continued need for efficient green light-emitting phosphors that do not emit UV radiation. It is also desirable to provide green light-emitting phosphors that use smaller amounts of expensive materials. It is also very desirable to use such novel phosphors to produce light sources having high energy efficiency and high CRIs.